This invention relates to oil well perforating and more particularly to high temperature detonators for use in oil well perforating with shaped charges in high temperature boreholes.
As oil wells have become deeper and more expensive to drill it has also become more difficult to perforate cemented casing in order to place the well on production due to the high temperature encountered near the bottom of boreholes in the deep wells. It is not uncommon in oil and gas production in the United States at at the present time to encounter wells from 16,000 to 22,000 feet in depth routinely which wells can have borehole bottom temperatures ranging from 300.degree. to 500.degree. F. dependent on the temperature gradient at the particular location of the well.
Conventional shaped charge perforating devices have relied upon detonators or initiators utilizing a primary explosive therein which may be detonated by heat or shock wave or a combination of both. Typically, an initiator charge or booster charge in a detonator will have a primary high explosive which may be heat initiated by a heating element which is activated by passing an electrical current therethrough. The electrical heating element heats the primary explosive in its immediate vicinity (usually in contact) and causes the detonation of this explosive which then propagates a shock wave through the surrounding explosive detonating it. The detonation of the initiator or detonator cap is then usually transmitted to a particular shaped charge explosive or explosive string by the use of detonating cord which connects the electricity activated initiator to each individual shaped charge in a string or gun which is being used to perforate a section of casing in a cased well borehole in order to provide entry ports for fluid production from the surrounding earth formations.
At the high temperatures encountered in wells being drilled and produced at the present time, however, the use of conventional primary explosives can become hazardous for two reasons. The explosives themselves become unstable at the elevated temperatures encountered near the bottoms of deep well boreholes. Also, there is always the possibility of forming stray electrical currents in the casing or the electrical wireline used for lowering the perforating gun into the section of casing to be perforated. Such currents can cause premature detonation of the very sensitive primary explosive where even small or slight electrical currents flow through the heating element of conventional detonators or blasting caps heretofore in common usage.
Because of the foregoing problems, it would be desirable to utilize all secondary explosives in the detonating caps or blasting caps for initiating the shaped charge explosives used for well perforating which are less sensitive to heat and other stimuli such as friction, sparks, impact, and static discharge. The more stable or less sensitive secondary explosives which can be obtained having relative heat immunity from degradation up to 600.degree. F. are more difficult to initiate or detonate than the conventional explosives in common usage in well perforating heretofore. The more thermally stable secondary explosives which are desirable for use in high temperature boreholes are more difficult to initiate. This is particularly so in the case of secondary explosives used in the blasting cap or detonator itself which may be utilized for this purpose. A safety feature which may be utilized in well perforating systems for use in high temperature boreholes therefore may comprise a relatively stable secondary explosive such as NONA (or generically 2,2',2",4,4',4",6,6',6"--nonanitroterphenyl). Alternatively, a high temperature stable secondary explosive such as HNS-1 (or generically 2,2',4,4',6,6'--hexanitrostilbene) could be used if desired.
Relatively stable secondary explosives such as NONA or HNS-1 may be detonated by the action of a type of detonator known as an exploding bridge detonator. In the exploding bridge type detonator, a conductive bridge having a relatively high current resistance is placed between two electrodes made of a good electrical conductor such as copper and which are connected to a source of electrical power. A relatively high voltage, short duration pulse of electrical energy is supplied to the two good conducting electrodes. The bridge portion of the device between the two relatively good current carrying electrodes is not capable of handling the high intensity short duration pulse of electrical energy and it heats rapidly and literally explodes from the passage of this current through it. The shock wave generated by the explosive of the bridging conductor is propagated through the surrounding medium of the secondary explosive material which is relatively stable at high temperature and used to initiate or detonate this secondary explosive. This technique thus avoids the problem of the use of heater type electrodes for initiation of the explosive material because a relatively higher intensity electrical pulse is required than could be accidentally obtained by the action of stray currents in the casing and wirelines used to support the well perforating instruments.
The use of relatively stable secondary explosives for the detonating cap or initiators has the disadvantage, however, that the relative difficulty of initiating this relatively stable secondary explosive in itself produces unreliable results when performed in the manner common to known prior art exploding bridge detonator type devices. The electrical impulses, foils and wire bridges previously used simply do not contain enough explosive capacity to reliably detonate a relatively stable secondary explosive such as NONA with acceptable reliability standards. An exploding bridge detonator device according to the concepts of the present invention can provide a safe and yet reliable configuration for the use of stable secondary explosives such as NONA as an initiator or some other equally or more stable secondary explosive.